Latest Developments

LINEAR is a new software package for simulating WFC3/IR grism observations at multiple rotation angles.

New source-dependent master sky images have been made available for the WFC3/IR grisms: ISR 2015-17 (Brammer et al., 2015). See ISR 2017-05 (Pirzkal, and Ryan 2017) for HeI information. Further information and downloads are available on the IR grism master sky images page.

Used for estimating the exposure time for WFC3/UVIS and WFC3/IR grism observations. For planning slitless spectroscopy with spatial scanning, please refer to Chapter 8.6 of the WFC3 Instrument Handbook and resources therein.

aXeSIM

We highly recommend that users utilize aXeSIM to simulate their WFC3 grism observations. In addition to the version available for download, a limited-feature version of aXeSIM is available as a web application.

II. Data Analysis

Detailed information on the data analysis of WFC3 grism data is provided in Chapter 9.4 of the WFC3 Data Handbook. This is the recommended starting point for all novice users.

CALWF3 Pipeline Processing

Data downloaded from the Mikulski Archive for Space Telescopes (MAST) is processed on the fly by the calwf3 pipeline with the most recent calibration files. Users generally should not need to reprocess their data. The links below provide more information on the pipeline.

Calibration files improve incrementally as we acquire more data and understand the instrument better. To find out if improved calibration files are available for an old data set, follow this guide.

WFC3 Reference Files

The most recent individual calibration files for WFC3 are available from the Calibration Reference Data System. These files are used by calwf3 and Astrodrizzle. Reference files can also be retrieved from MAST or downloaded directly from the iref directory.

aXe Spectral Extraction Software

The calwf3 processing of the grism observations does not include extraction or calibration of the spectra. This must be done off-line using additional tools. The current software supported by STScI is aXe, which was developed by the Space Telescope European Coordinating Facility (ST-ECF). This package enables automatic and reliable extraction of large numbers of spectra from individual images.

Installation

aXe can be installed as part of Anaconda 3. It requires a Python 2.7 installation, Pyraf , pydrizzle and multidrizzle. It is therefore recommended that i be installed as a new Anaconda 3 environment, using the following steps:

Once the axe-wfc3 is activated (in a new terminal, after issuing the command “source activate axe-wfc3), add pydrizzle and multidrizzle to axe-wfc3:
conda install pydrizzle
pip install multidrizzle

You will likely need Sextractor, which can be installed using anaconda with:
conda install sextrcator

Data Reduction

The WFC3 IR Grism Cookbook is a step-by-step guide to reducing G141 IR grism data using aXe. The procedures described in the Cookbook are directly applicable to the G102 grism with small modifications. The grism data from the WFC3 ERS Program used as an example in the Cookbook is also available for download.

An UVIS Cookbook is not currently available. In the interim, ISR 2011-18 provides helpful suggestions on using aXe to reduce G280 data.

Visualization

aXe2web is a software tool which produces an html summary of each reduced object such as in this example. Documentation and software can be found here.

The configuration and calibration files used with the aXe extraction software and the aXeSIM simulation software are available for download below. The download for each grism includes sensitivity, flat-field cube, and configuration files for different direct filter - grism combinations. To use these calibration products with aXe and aXeSIM, copy them into a local directory and set your AXE_CONFIG_PATH environment variable to point to that directory. No further re-formatting is needed.

G280 UVIS Grism

All files: WFC3.UVIS.G280.cal.tar.gzInformation on the derivation of the G280 calibration files as well as individual file downloads are available here.

Persistence

The WFC3 IR detector is susceptible to persistence which appears as afterglow in parts of the detector that experienced high fluence (total number of photo-electrons released) in earlier exposures. Persistence is not corrected for in calwf3 or aXe. Examples of persistence and suggestions on how to mitigate it are given in the WFC3 Data Handbook.

STScI provides a persistence model for each WFC3 IR dataset
through a separate persistence search form which allows users to view and download the persistence model. Search for the dataset of interest and then follow the links under "Dataset" to view the model and the links under "Visit" to download the model. A full description of the data products is provided here. A detailed description of the current modeling algorithm can be found in ISR 2015-15.

Observers and archive users are encouraged to contact the help desk if the persistence subtraction substantially limits their ability to extract science from an image or set of images.

IR Grism Master Sky Images

Master sky images have been provided for both G102 and G141, however the single images used by aXe do not take into account the full complexity of the sky background of grism observations. A more accurate background subtraction can be achieved by using separate images for each of the background components: zodiacal light, He I emission and scattered light (for G141). Further information and downloads are available on the WFC3 grism master sky page and in WFC3 ISR 2015-17. The use of the new sky images has not been implemented in aXe.

To take advantage of the new sky images, users are encouraged to follow the algorithm described in the appendix of WFC3 ISR 2015-17.

III. Additional Resources

LINEAR is a suite of IDL and C routines designed to simulate WFC3/IR grism spectroscopy observations at multiple positions/rotation angles. Most packages extract spectra for individual ORIENTs and stitch the contamination-corrected 1-d spectra (for each roll angle) post facto. This is the fundamental distinction with LINEAR, where we attempt to solve for the non-parameteric spectrum that best fits the entire canon of available dispersed images (that may be rotated or dithered with respect to each other). 1-dimensional spectral extraction tools are in development. The code will be described in forthcoming publications by R. Ryan & S. Casertano.

WAYNE is a WFC3 IR Grism Spectroscopy simulator described in Varley & Tsiaras (2015). It can simulate both staring and spatial scanning observations. Currently the code is not public but requests for access can be sent to the author.

Webpage for the 3D-HST Survey - a spectroscopic galaxy evolution survey with HST. 3D-HST has obtained G141 grism spectroscopy over four of the five CANDELS fields (AEGIS, COSMOS, GOODS-S and UDS). HST and ground-based images, photometric catalogs and grism spectra for all five CANDELS fields are available for download from the team website. The data reduction pipeline employed by 3D-HST is described in Momcheva, van Dokkum, Brammer et al. (2015)

The 2010 workshop highlighting scientific results of slitless spectroscopy and tutorials on using and analyzing data from the WFC3 Grism. A direct link to the archived videos of the talks can be found here.

Homepage of PandExo: A community tool for transiting exoplanet science with JWST & HST

IV. Grism-Related Instrument Science Reports

Instrument Science Reports (ISRs) are STScI in-house refereed technical papers which address specific topics related to instrument and telescope performance or data analysis techniques. A full list of WFC3 instrument science reports (ISRs) can be found here. This grouping of ISRs has been compiled to provide additional information to observers beyond what is found in the Instrument Handbook.

Investigates the cause of strong time-dependent variations of the background levels observed in a variety of filters of the infrared (IR) channel of the WFC3 as due to metastable helium emission at 10,830 Angstroms. Characterizes the effect on archival observations and recommends mitigation strategies.